Stephen Hawking, the most famous physicist of his time, has died at the age of 76.

A family spokesman said Hawking, who was British, died peacefully early Wednesday at his home in Cambridge, England.

Hawking was a theoretical physicist. He was known worldwide for working to explain subjects like the beginnings of the universe and the complexities of black holes.

He was diagnosed with a disease called amyotrophic lateral sclerosis, or ALS, at age 21. At the time, doctors predicted he would only live a few years. But he proved them wrong by surviving for more than 50 years more and continuing his scientific work.

The disease eventually put him in a wheelchair and took away his ability to speak. For many years, Hawking communicated by using a voice synthesizer.

In this March 6, 2017, file photo, Stephen Hawking receives the Honorary Freedom of the City of London. Cambridge University has put Hawking's doctoral thesis online "Properties of Expanding Universes" online.

In his 2013 book, “My Brief History,” Hawking wrote about first learning of the illness: “I felt it was very unfair - why should this happen to me?” He added: “At the time, I thought my life was over and that I would never realize the potential I felt I had.” “But now, 50 years later, I can be quietly satisfied with my life,” he wrote.

Hawking was one of Sir Isaac Newton’s successors as the Lucasian Professor of Mathematics at Cambridge University. In this role, he was involved in the search for one of the great goals of physics – to find a “unified theory.”

He studied Albert Einstein's General Theory of Relativity, which describes the movement of large objects. He sought to connect that theory with the Theory of Quantum Mechanics, which deals with subatomic particles.

"My goal is simple,” Hawking once said. “It is a complete understanding of the universe, why it is as it is and why it exists at all."

His 1988 book "A Brief History of Time" became an international bestseller and brought him widespread fame.

Hawking was also recognized for his successful research on black holes. He was able to prove that small amounts of radiation could escape black hole gravitational pull. His work led the discovery to become known as Hawking radiation.

A sign of his great popularity came in October 2017, when Cambridge put Hawking's 1966 thesis on the internet for the first time. Demand for the thesis was so high that it caused the university's website to go down.

Hawking said belief in a God who intervenes in the universe “to make sure the good guys win or get rewarded in the next life” was wishful thinking. “But one can’t help asking the question: Why does the universe exist?” he said in 1991. “I don’t know an operational way to give the question or the answer, if there is one, a meaning. But it bothers me.”

Stephen Hawking, left, is joined by a group of of scientist including Princeton physicist Freeman Dyson, right, announce the new Breakthrough Initiative focusing on space exploration and the search for life in the universe, during a press conference, Tuesday, April 12, 2016, at One World Observatory in New York. (AP Photo/Bebeto Matthews)

Hawking was a big supporter of human space travel to the Moon and Mars. He said such missions would help unite humanity in a shared purpose of spreading the human race beyond Earth.

"We are running out of space and the only places to go to are other worlds. It is time to explore other solar systems. Spreading out may be the only thing that saves us from ourselves. I am convinced that humans need to leave the Earth," he said last year.

I’m Bryan Lynn.

Bryan Lynn wrote this story for VOA Learning English, based on reports from VOA News, the Associated Press and Reuters. Mario Ritter was the editor.